1. Field of the Invention
The present invention relates to an optical waveguide device such as an arrayed waveguide grating (AWG), and a fabricating method thereof, and in particular, to an optical waveguide device and a fabricating method thereof suitable for cutting or processing a portion formed with a light guiding core.
2. Description of the Related Art
With the remarkable development of communications technology such as the Internet, it has rapidly been required that optical waveguide devices be enhanced in function, and hybrid packaging of optical waveguide devices has also been actively performed. Such hybrid packaging requires high-precision cutting of the device at a predetermined portion for joining the optical waveguide devices, or each kind of processing such as provision of a groove or space in the device for incorporating other components. Also, even when such hybridization is not performed, components such as optical fiber arrays are often joined to the end face of the optical waveguide device for light input/output. To this end, cutting of the end face or predetermined portion of these optical waveguide devices cut out from the wafer is performed.
However, when such cutting of the optical waveguide devices is performed, chippings and recesses occur in the end face of the optical waveguide in the event that its upper surface is uneven. Consequently, the optical connection with a component arranged in contact with the end face is not satisfactorily performed. Prior to the examination of the shape of the cut surface, the occurrence of unevenness in the upper surface of the optical waveguide device is explained.
FIG. 1 illustrates an example of a production process when a core and a cladding are formed over a substrate. First, as illustrated in FIG. 1A, substrate 1 such as silicon (Si) is prepared. Then, as illustrated in FIG. 1B, substrate 1 is coated on one side with lower cladding layer 2. Further, as illustrated in FIG. 1C, lower cladding layer 2 is coated with core layer 3.
FIG. 1D illustrates a processing step for removing an unwanted portion of core layer 3. Core layer 3 is applied with photoresist 4, covered with photo-mask 5 and irradiated with ultraviolet light 6. Pattern 7 matched with a portion which remains as a core is formed in photo-mask 5, so that in photoresist 4 only an area where the portion remains as the core, or an area except the portion which remains as the core is irradiated with the ultraviolet light. FIG. 1E illustrates a state subsequent to the development of photoresist 4. Photoresist 4A is only the portion which remains as the core.
FIG. 1F illustrates a state subsequent to etching. The etching removes a portion of core layer 3 on which photoresist 4A is absent, so that only required core portion 3A remains. FIG. 1G illustrates a state subsequent to the removal of photoresist 4A by a chemical.
Thereafter, as illustrated in FIG. 1H, lower cladding layer 2 and core portion 3A are coated with upper cladding layer 8. In this coating process, since the portion to be the cladding is deposited from the top of this diagram, upper cladding layer 8 is higher on the portion corresponding to protruding core portion 3A than on the other portion thereof in FIG. 1G.
FIGS. 2-4 illustrate some of cross-sectional structures of optical waveguide devices. In these figures, the same characters as FIG. 1 denote like portions. FIG. 2 illustrates an optical waveguide device produced by the production process explained in FIG. 1, and which is called the embedded waveguide.
FIG. 3 illustrates an optical waveguide device called the ridge waveguide, in which portion 21A of core layer 21 formed on cladding layer 2 formed on substrate 1 protrudes. Portion 21A serves as the core. FIG. 4 illustrates an optical waveguide device called the strip loaded waveguide, in which core layer 22 formed on cladding layer 2 is partially loaded with core 23.
Although there are some other different production processes for optical waveguide devices, cores 3A, 21A, and 23 of the optical waveguide devices illustrated in FIGS. 2-4 are formed even higher than the periphery. Therefore, forming upper cladding layer 8 in the following step makes the portion on cores 3A, 21A, and 23 higher than the other portion.
The prior-art processing will hereinafter be explained concerning the cutting of such optical waveguide devices.
FIGS. 5 and 6 illustrate an example of forming a groove by cutting halfway through a substrate of an optical waveguide device. This proposal shown in Japanese unexamined patent publication No. 11-23873 processes the groove in the optical waveguide device. As illustrated in FIG. 5, the 1st step forms groove 16 by etching in waveguide 15 made of core 12 and claddings 13 and 14 formed on substrate 11. As illustrated in FIG. 6, the 2nd step also forms groove 16 in substrate 11 by mechanical cutting with narrower blade 17 than groove 16.
This proposal forms the groove against hard waveguide 15 by etching in the 1 st step. Accordingly, damage to blade 17 is reduced. Also, since groove 16 is formed in waveguide 15 by etching, the chipping of waveguide 15 is reduced. Also, the positional control of blade 17 is disclosed in Japanese unexamined patent publication No. 2000-275450.
Now, the core and claddings are present in the waveguide on top of the optical waveguide device, and the unevenness is present between the core and claddings and the upper surface portion of the substrate being positionally matched thereto, as explained above. Cutting such an uneven portion or groove therein causes deformation in the shape of the processed end face, and degradation in the optical characteristics.
FIG. 7 illustrates a near end of an optical waveguide device, while FIG. 8 is a top view of the optical waveguide illustrated in FIG. 7. Optical waveguide device 31 shown in these figures is made of substrate 32 and waveguide 33, wherein waveguide 33 protrudes upwards. Optical waveguide device 31 is cut in a slightly inner portion from one end along broken line 35 perpendicular to the top surface of the substrate (see FIG. 8). Cutting techniques by reactive ion etching and by a blade are examined.
FIG. 9 illustrates an outline of a production process by reactive ion etching. FIG. 9A illustrates a side view of optical waveguide device 31 prior to processing. Optical waveguide device 31 is applied with photoresist 41 (FIG. 9B), covered with photo-mask 42, and irradiated only on broken line 35 with ultraviolet light 43 (FIG. 9C) for pattern transfer. Then by the development of photoresist 41, photoresist 41 is selectively removed with respect to the UV-irradiated line.
Thereafter, a gas is changed by the application of high frequency power into a plasma state to produce accelerated plus ions to collide with the optical waveguide device, and thereby cause reactive ion etching (RIE). The introduced gas uses a compound containing a halogen such as fluorine or chlorine to be reactive with the substrate material and tend to produce a volatile substance. Accordingly, the gas and substance in the cut location react to produce a volatile substance, while the processing progresses.
FIG. 10 illustrates a manner of cutting an optical waveguide device with a blade. In this case, optical waveguide device 31 is cut along broken line 35 (see FIG. 8) with disk-shaped blade 51 pressed thereagainst.
FIGS. 11A and 11B illustrate a change in the end face subsequent to reactive ion etching. Waveguide 33 protruding from substrate 32 is recessed (recess 36) from the other cut surface. This recess 36 is caused by thinner photoresist 41 on protruding waveguide 33 than on the other top surface as illustrated in FIG. 9C. That is, since protruding waveguide 33 tends to be etched more than the other portion, the cut portion of protruding waveguide 33 is recessed.
On the other hand, the cutting of the optical waveguide device by blade 51 (shown in FIG. 10) or dicing saw tends not only to damage the blade at the relatively hard protruding core portion, but also to cause chipping in waveguide 33, as pointed out in Japanese unexamined patent publication No. 11-23873.
FIG. 12 illustrates an example of such chipping in a waveguide. Chipping 37 occurs at the end of protruding waveguide 33 from substrate 32 in FIG. 12 corresponding with FIG. 11A. Also, in the optical waveguide device where the core forms the protrusion while the cladding forms the recess as illustrated FIGS. 3 and 4, local stress concentration tends to selectively cause chipping in the core or cladding. Blade deformation by such chipping and protrusion causes a defect in the cut end face.
As explained above, in the case that the upper surface of the optical waveguide device is uneven, the prior art has difficulty in high-precision cutting thereof. This is also true for the case of the processing of a groove. Consequently, the problem exists of being unable to satisfactorily bond or incorporate other components to be matched with the processed end face and groove, and degrading the optical characteristics of hybrid-packaged components or components joined to other components at the end face.